1. Field of the Invention
This invention relates to a motion picture camera, and more particularly to an exposure time prolonging means provided in a motion picture camera of the type which employs a rotary shutter blade comprising a sector means.
2. Description of the Prior Art
When using a conventional motion picture camera which is designed to take normal motion pictures with a predetermined exposure time and at a preselected frequency (such as 18 or 24 frames per second), an illuminating device must be used in twilight, at night or under other circumstances when the intensity of scene light is low so as to obtain motion pictures of normal quality. However, it is undesirable to use an illuminating device from the viewpoint of the quality of pictures since it is often desired to obtain delicate shade distribution and soft contrast of the scene. Further, it is troublesome to carry and operate the illuminating device together with the motion picture camera.
In view of the above-mentioned drawbacks of the conventional motion picture cameras, it has been proposed to design a motion picture camera which is able to obtain substantially bright motion pictures under circumstances when the intensity of scene light is low. There are two ways to make it possible for a motion picture camera to obtain normal motion pictures under low intensity of light. One is to lower the F-number of the taking lens of the camera and the other is to prolong the exposure time.
The exposure time of the motion picture camera can be prolonged either by enlarging the angle of opening of the sector of the rotary shutter blades or lowering the frequency of the frame feed. The frequency or the frame speed of the film feed, however, must be higher than the standard frame speed 18fr/sec, and accordingly, the frame speed is subject to a lower limit. Therefore, the best way to prolong the exposure time is to enlarge the angle of opening of the sector of the rotary shutter blades.
The enlargement of the angle of opening of the sector of the rotary shutter blades results in reduction in the angle of the non-opening portion of the sector portion of the shutter. Therefore, in order to enlarge the opening angle of the shutter, the claw which intermittently pulls down the film must be quickly moved so that the claw may feed one frame of the film while the light coming through the taking lens is intercepted by the non-opening portion of the sector.
In the mechanism for performing the above operation of the claw, there is a great problem caused by the difference in the characteristic of the motion (non-uniform cyclic motion) of the claw and that of the motion (uniform cyclic motion) of the control cam for controlling the motion of the claw. For instance, in conventional cameras in which a so-called triangular cam is used for controlling the claw, the claw is driven by a claw controlling mechanism which creates an abruptly changing acceleration. Owing to the abruptly changing acceleration, the image stability on the film is lowered.